1. Field of the Invention
The present invention relates to a system for display images including thin film transistor (TFT) liquid crystal display (LCD), and more particularly relates to a TFT array substrate.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) comprises a lower substrate, an upper substrate and a liquid crystal layer interposed therebetween. The upper substrate typically comprises a color filter and a common electrode. The lower substrate typically comprises a plurality of pixel areas defined by crossing gate lines and source lines (or data lines). Each pixel area comprises a thin film transistor serving as a switching element located near the intersection of the gate and source lines and a pixel electrode electrically connected to the thin film transistor. The thin film transistor comprises a gate, a source and a drain, wherein the gate is typically an extended portion of the gate line and the drain is electrically connected to the pixel electrode via a contact hole. The liquid crystal layer interposed between the common electrode and the pixel electrode forms a capacity of liquid crystal.
In order to improve image quality, a storage capacitor is formed in each pixel area. FIG. 1 shows the essential components of a TFT array substrate used in a conventional thin film transistor liquid crystal display (TFT-LCD). Typically, the formation process of the structure shown in FIG. 1 includes the following steps: forming a first active layer 2a, a second active layer 2b in a driver area 16, a third active layer 2c and a bottom electrode 2d in a pixel area 22; performing an implantation to form source/drain regions 5 in the first active layer 2a of the driver area 16, source/drain regions 15 in the third active layer 2c of the pixel area 22 and an n+ type doped bottom electrode 2d in the pixel area 22; depositing a first gate dielectric layer 3 covering the first active layer 2a, the second active layer 2b, the third active layer 2c and the bottom electrode 2d; depositing a second gate dielectric layer 7 on the first gate dielectric layer 3; and forming a first gate 6, a second gate 8, a third gate 10, and a top electrode 14 overlying the second gate dielectric layer 7 of the first active layer 2a, the second active layer 2b, the third active layer 2c and the bottom electrode 2d. 
Nevertheless, the conventional LCD has some technological issues. For example, since the storage capacitor is opaque, the aperture ratio of a pixel is reduced resulting inferior pixel quality. Additionally, since the dielectric layers of the capacitors (gate capacitor and storage capacitor) are made from the same material and are of same thickness, the capacitance value per unit area for the gate capacitor and storage capacitor is same. Thus, the capacitance of the gate capacitors and storage capacitors can not be independently controlled and the value of storage capacitance per unit area is dependent on the thickness requirements of the gate dielectric. Moreover, the method hinders thinning of the dielectric layer.
What is needed, therefore, is a TFT array substrate with a storage capacitor having a thinner dielectric layer for a TFT-LCD.